Research and development efforts are made for the applications of the resonant tunneling phenomenon to high-speed semiconductor devices as well as new functional devices. Since each electron merely consumes an extremely short transit time period for traveling over the base layer by the agency of the resonant tunneling phenomenon, an extremely high switching speed can be achieved by the resonant tunneling bipolar device. Another attractive point of the resonant tunneling bipolar device resides in the fact that a negative differential resistance region takes place in the base-emitter current-to-voltage characteristics, and the negative differential resistance is conducive to the high speed switching operation as well as providing a potentiality for a new function to the semiconductor device.
A typical example of the resonant tunneling hot electron transistor is disclosed by Yokoyama et al in "A New Functional, Resonant-Tunneling Hot Electron Transistor (RHET)", Japanese Journal of Applied Physics, Vol. 24, No. 11, November, 1985, pages L853 to L854. Yokoyama et al propose the structure a half of which is shown in FIG. 1, and the energy band diagram shown in FIG. 2 takes place in the structure under an appropriate biasing state. For better understanding of the Applicant's invention, description is hereinunder made for the structure of a typical example of the resonant tunneling hot electron transistor the structure and the behavior thereof.
Referring first to FIG. 1 of the drawings, the resonant tunneling hot electron transistor is fabricated on a semi-insulating substrate 1 of gallium arsenide. On the gallium arsenide semi-insulating substrate 1 are successively grown an n-type gallium arsenide collector layer 2, a non-dope aluminum-gallium-arsenide potential barrier layer 3, an n-type gallium arsenide base layer 4, a non-doped aluminum gallium arsenide potential barrier layer 5, a non-doped gallium arsenide quantum well layer 6, a non-doped aluminum gallium arsenide potential barrier layer 7, and an n-type gallium arsenide emitter layer 8 which form in combination a multiple-layer structure partially cut away to expose the collector layer 2 and the base layer 4. A collector electrode 9 and a base electrode 10 are provided on the exposed surfaces of the base and collector layers 2 and 4, respectively, and the emitter electrode 11 is formed on the emitter layer 8. The non-doped aluminum gallium arsenide quantum well layer 6 and the non-doped aluminum gallium arsenide potential barrier layers 5 and 7 as a whole constitute a super-lattice film structure 12 for a quantum well resonator.
In the structure shown in FIG. 1, an energyband takes place in a biasing state as shown in FIG. 2. With an appropriate voltage level applied between the base electrode 10 and the emitter electrode 11, the quantum well resonator injects electrons into the n-type gallium arsenide base layer 4 due to the resonant tunneling phenomenon, and each of the electrons injected into the base layer 4 becomes a hot electron HE traveling over the n-type gallium arsenide base layer 4 at an extremely high speed before reaching the n-type gallium arsenide collector layer 2. A negative differential resistance region is achieved in the base-emitter current-to-voltage characteristics due to the resonant tunneling phenomenon, and each of the hot electrons merely consumes an extremely small amount of time period, so that the resonant tunneling hot electron transistor is an attractive candidate for the component element of an high speed switching circuit such as, for example, a flip flop circuit. An example of such a high-speed flip-flop circuit is illustrated in FIG. 3. In detail, the high-speed flip-flop circuit is provided with the resonant tunneling hot electron transistor 21 coupled between a source of positive high voltage level Vcc and the ground, and resistors 22 and 23 are provided between the base electrode and an input signal terminal 24 and between the collector electrode and the source of positive high voltage level Vcc, respectively. An output terminal 25 of the flip-flop circuit 21 is provided between the resistor 23 and the resonant tunneling bipolar transistor 21. If an appropriate biasing voltage level is applied to the input terminal 24, the flip-flop circuit is responsive to an input signal S of either positive or negative voltage level with respect to the biasing voltage level and shifted between two stable states due to the base-emitter current-to-voltage characteristics of the negative differential resistance. Thus, the flip-flop circuit is implemented by using the single resonant tunneling hot electron transistor 21.
The flip-flop circuit shown in FIG. 3 is simple in circuit arrangement, however, a problem is encountered in application of the flip-flop circuit to a complex circuit such as, for example, a counter circuit or a shift resistor. In general, it is easy to form a complex circuit with flip-flop circuits each producing not only an output signal but also the complementary output signal thereof. However, the flip-flop circuit shown in FIG. 3 merely produces the single output signal, and, for this reason, the flip-flop circuit shown in FIG. 3 is combined with an inverter circuit for producing the complementary output signal. A difference in phase takes place between the output signal and the complementary output signal due to the switching time period of the inverter circuit, and the interconnection between the flip-flop circuit and the inverter circuit is causative of another time delay. These retard a signal propagation in the complex circuit.